1. Field of the Invention
The present invention relates to the process of a setting calcined gypsum slurry, and is more particularly concerned with a composition for reducing the amount of water necessary in forming a pourable aqueous calcined gypsum slurry to use in making cast or molded products.
2. Prior Art
Plaster (i.e. calcined gypsum) has long been a large volume commercial article of commerce either as various dry plaster compositions to be mixed by the user with water in forming cast gypsum articles or in manufactured products such as gypsum wallboard and cast art objects and the like. Generally, molds or cast gypsum articles are manufactured by dispersing calcined gypsum and additives in sufficient water to form a pourable slurry, casting the slurry into a mold of desired shape, allowing the slurry to set, and drying excess water from the set article.
The gypsum setting reaction involves the reaction of calcium sulfate hemihydrate and water to form calcium sulfate dihydrate. The theoretical water required to convert the calcined gypsum to set gypsum dihydrate is only 18.7% by weight on a pure basis. However, considerable excess water is required to fluidize the calcined gypsum and obtain proper flow of the gypsum slurry in the casting or molding operation. The amount of excess water depends primarily upon the type of plaster particle, whether beta hemihydrate or alpha hemihydrate, and other additives in the plaster composition. Additives conventionally used in minor amounts include accelerators, retarders, fibrous reinforcements, and consistency reducers. Consistency reducing agents are typified by the lignosulfonates, gum arabic, modified starches and other cellulosic derivatives. Ordinarily, plasters produced at different locations from different sources will also vary in their water requirements to form a pourable slurry. Thus, calcined gypsum compositions that are predominantly or substantially beta hemihydrate may have a normal consistency in a range of about 60-90 or more cc. This relates to a water usage of 60-90 cubic centimeters of water per 100 grams of the calcined gypsum in order to form a readily pourable and flowable gypsum slurry. Other calcined gypsum compositions that predominantly contain alpha hemihydrate may have a normal consistency of 34-55 cc. for example.
The use of cellulosic additives as consistency reducers in calcined gypsum plasters has been suggested as early as around 1900.
In the area of another type of inorganic cementitious material, Portland cement, various other materials have been suggested as additives to modify properties of the composition. For example U.S. Pat. No. 3,465,825 suggests the use of the mixed salts of lithium and sodium of the condensation products of mononapthalene sulfonic acid and formaldehyde as fluid loss control agents in Portland cement compositions. Such compositions are highly alkaline, generally pH 11 and higher, and the use of such additives have been thought to be effective only in highly alkaline media. For certain specific industrial applications it has been possible to combine substantial amounts of Portland cement compositions and plaster compositions. See for example U.S. Pat. Nos. 3,582,376; 3,847,635 and 3,852,081. Such compositions because of their high alkalinity due to the substantial quantities of the Portland cementitious ingredients have taught the utilization of naphthalene sulfonic acid and formaldehyde condensation product consistency reducing additives. However, the available materials of this type have not been entirely effective or satisfactory in non-highly alkaline media and their usage has not been recommended in plaster compositions which have not been adjusted to high alkalinity. In addition attempts to utilize commercially available condensates of this type in plaster compositions has revealed a severe detrimental characteristic of the condensate. Compositions containing substantial amounts of Portland cements are generally a dark grey in color; however, predominantly plaster containing compositions, especially those most usable in the manufacture of pottery and artware objects are very light in color and present a quite pleasing appearance. The addition of commercially available sodium form of naphthalene sulfonic acid and formaldehyde condensates resulted in a brown discoloration developing in the cast article on exposure to sunlight and with time, which discoloration is highly objectionable from a marketing standpoint.